var e = document.createElement('p');
var t = document.createTextNode("My content goes here");
e.appendChild(t);
document.body.insertBefore(e, document.body.childNodes[0]);
Once you have created the element e, you have a javascript object on your hands.
You either access its innerHTML property (first example), or build its content with appendChild (second example).
Both are dynamic:
e.innerHTML = e.innerHTML + '<span>More of my content</span>'
var s = document.createElement('span');
var t = document.createTextNode("More of my content");
s.appendChild(t);
e.appendChild(s);